1. Field of the Invention
The present invention generally relates to methods for electroplating a metal onto a plastic substrate. More particularly, this invention relates to a method for electroplating polyphthalamide compositions in which an impact-enhancing elastomeric material is dispersed.
2. Description of the Prior Art
Motor vehicles often include a substantial number of chrome-plated trim elements for both decorative and functional purposes. Such trim elements include bumpers, body side moldings, lower body claddings, wheel covers and grilles as well as other components. The overall appearance of the vehicle is significantly enhanced by these highly reflective chrome surfaces. However, trim elements also serve a functional purpose in that they help to absorb impact when the vehicle is involved in a collision and when the vehicle contacts flying gravel, road debris, roadway abutments or the like. Accordingly, plated metal on a trim element preferably must withstand impact without chipping, cracking, or delaminating.
Traditionally automotive trim elements have been manufactured from materials such as chrome-plated steel, anodized aluminum and stainless steel, which are capable of absorbing relatively high impacts. However, such traditional trim elements not only add significantly to the cost of the vehicle, but also substantially increase the vehicle weight thereby decreasing fuel economy. Therefore, there has been considerable effort by the automotive industry toward developing a cost effective, lightweight alternative to the traditional trim materials, and particularly chrome-plated metals. Plateable plastics are such a desirable alternative, because they not only reduce the vehicle weight, and thereby correspondingly increase the vehicle fuel economy, but plateable plastics also allow for part consolidation within the automobile. Plastics have much greater design flexibility than metals, in that they can be easily molded into a variety of complex and contoured configurations that cannot be achieved with conventional metal stamping and forming operations. In addition, when components are formed from plastic materials, a significant cost savings can be realized over comparable components formed from metal.
A wide variety of plated plastics are known. For example, acrylonitrilebutadiene styrene (ABS) has been plated to provide decorative articles such as headlamp surrounds, and plumbing and marine hardware, and polycarbonates (PC) have been utilized as the substrate for plated motor vehicle door handles. In addition, several other plastics have been successfully plated for various decorative purposes. However, these plastics, though plateable for decorative purposes, do not generally provide a satisfactory substrate if the finished article must be capable of sustaining significant impact or endure hot/cold thermal cycling. Accordingly, the use of plated ABS and PC plastics on an automobile has been limited because the plated metal is prone to delamination if the plastic is subjected to low energy impact. In addition, as an extreme example, these plastic materials may even shatter upon impact.
Thus, for a plastic to be suitable as a substrate for a plated component of an automobile, it must generally have sufficient impact and thermal cycle resistance. Furthermore, its impact resistance must be acceptable over a wide range of temperatures. Attempts to develop impact-resistant plastics have resulted in the formulation of many blends, including acrylonitrile-butadiene-styrene (ABS)-modified polycarbonate resins as taught in U.S. Pat. No. 5,846,665 to Timmer et al. (assigned to the assignee of the present invention), and polyphthalamide (PPA) resins commercially available under the trademark AMODEL.RTM. from Amoco Performance Products, Inc. PPA resins are characterized by high strength at elevated temperatures, e.g., a heat deflection temperature (HDT) of about 380.degree. F. to about 400.degree. F. (about 193.degree. C. to about 204.degree. C.) at about 266 psi (about 18.3 bar) (ASTM D648). PPA resins can be processed to contain various reinforcement fibers and fillers to promote their mechanical and physical properties. In particular, PPA resins modified with up to 15 weight percent dispersed elastomer have been found to exhibit excellent impact resistance.
The present literature indicates that articles formed of mineral-filled PPA resins can be metallized by electroless and electroplating techniques. In particular, if the PPA resin has been specifically formulated for electroless plating, conventional electroless techniques developed for ABS resins can be used to develop a thin layer of metal. Such techniques have generally entailed etching with a chromic acid-sulfuric acid etch solution, then treating the etched surface with a neutralizing solution, a catalyzing solution of a noble metal salt solution, an acid accelerating solution, and then an electroless plating solution. The electroless metal layer, typically copper, can serve as a functional plating, e.g., EMI/RFI shielding, or as the cathode for a subsequent electroplating process to yield a decorative finish on the article.
However, attempts to plate certain modified PPA compositions have been plagued with poor metal adhesion and/or blistering. More particularly, unfilled impact-modified PPA resins have been determined to be very difficult to successfully plate, defined herein as the resulting metal plating having a peel strength of greater than about two pounds (about 8.9 newtons). Employing conventional ABS processing, metal plating exhibits unreliable plate deposition and adhesion. Consequently, skip (areas without coverage) and blistering result in an unacceptable product. At best, the metal plate blisters with thermal cycle testing. While various methods have been suggested for improving adhesion between plated metal and a plastic subs ate, such as pre-etch conditioning as taught in U.S. Pat. No. 4,125,649 to Donovan et al., such techniques have not solved the problem of poor adhesion of plated metal to an article formed of an elastomer-modified PPA resin.
Accordingly, what is needed is a process for electroplating metal to an impact-resistant, high temperature PPA plastic, particularly an elastomer-modified PPA substrate, wherein the resulting product is characterized by excellent adhesion between the metal plating and the PPA substrate.